1. Field of the Invention
The present invention relates to a method of manufacturing a silica glass crucible for pulling silicon single crystals. In particular, the present invention relates to a method of manufacturing a silica glass crucible for pulling silicon single crystals permitting the manufacturing of a silica glass crucible that has a smooth inner surface and that is capable of producing silicon single crystals at high yield when used to pull silicon single crystals.
2. Discussion of the Background
The silicon single crystals that are employed as the substrates of semiconductor devices are generally manufactured by the Czochralski method (CZ method). In the CZ method, a polycrystalline silicon starting material is charged to a silica glass crucible for pulling silicon single crystals, the silicon starting material is peripherally heated and melted, and a seed crystal suspended from above is brought into contact with the silicon melt and pulled.
Conventional silica glass crucibles are formed by a method such as fusion under reduced pressure. With employing a silica glass crucible fabricated by this method, bubbles are generated in the transparent layer when a high temperature is reached during silicon single crystal pulling, and the silica glass crucible deforms and is corroded from the surface by the silicon melt. This corrosion exposes bubbles in the transparent layer (inner layer) of the silica glass crucible at the interface with the silicon melt. Single crystallization becomes unstable, resulting in a problem in the form of a diminished single crystallization yield.
Specifically, when minute bubbles are present near the inner surface of a silica glass crucible, those bubbles that are present near the opening of the silica glass crucible expand during pulling of a single crystal, splitting open and dropping into the melt, or coming into contact with the silicon melt in the silica glass crucible, resulting in portions containing bubbles being abruptly damaged. These phenomena are thought to diminish the single crystallization yield of silicon single crystals.
Accordingly, various manufacturing methods that eliminate bubbles in the transparent layer of the silica glass crucible have been examined in recent years. Silica glass crucibles are normally manufactured by the rotating arc fusion method under reduced pressure (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-160836 (D 1)). Various improvements in this manufacturing device and changes in the silica starting material have been investigated, but the complete elimination of bubbles has yet to be achieved.
Accordingly, a method for manufacturing a silica glass crucible for pulling silicon single crystals has been proposed that permits the manufacturing of a silica glass crucible with an inner surface from which residual bubbles have been completely eliminated and cannot be found by visual or microscopic examination; in which bubble nuclei that generate bubbles during use are substantially absent in the vicinity of the inner surface; and in which the inner surface is smooth and yields a high crystallization rate (DF rate) (Japanese Unexamined Patent Publication (KOKAI) No. 2001-002430 (D 2)).
This method is characterized in that the entire inner surface of a silica glass crucible is mechanically ground and the inner surface is fused a second time by arc fusion. The silica glass crucible is one manufactured by a method of manufacturing a silica glass crucible for pulling silicon single crystals by feeding a quartz starting material powder into a rotating mold to form a shaped molded product and by arc-fusing the molded product to give a crucible. D 2 states that this method permits the manufacturing of a silica glass crucible with an inner surface from which residual bubbles have been completely eliminated and thus cannot be found by visual or microscopic examination, in which bubble nuclei that generate bubbles during use are substantially absent in the vicinity of the inner surface, and in which the inner surface is smooth and yields a high DF rate.
As an example of a manufacturing method that eliminates bubbles from the transparent layer of a silica glass crucible, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-157427 (D 3) describes a method of manufacturing a silica glass crucible by feeding a quartz starting material powder into a mold that is permeable to air; forming a crucible-shaped molded product; reducing the pressure; and feeding hydrogen gas, helium gas, or a mixed gas thereof at the beginning of fusion. Based on this method, the hydrogen and helium gases diffuse into the transparent layer of the silica glass crucible, gases other than these gases do not diffuse, and the gases in the bubbles that are formed during fusion by heating can be eliminated by diffusion to the exterior from within the quartz glass. However, the same problems as in the above-described manufacturing method are encountered when manufacturing a silica glass crucible by feeding helium gas from the start to the end of fusion. When hydrogen gas or a mixed gas of hydrogen and helium gas is fed from the start to the end of fusion to manufacture a silica glass crucible, although the expansion of gas bubbles can be inhibited during the pulling of a silicon single crystal, a layer of minute bubbles of about 1 mm remains in the outer transparent layer and the number of bubbles is unlimited and thus cannot be made to approach zero.
A method of manufacturing a silica glass crucible for pulling silicon single crystals that is characterized by feeding a quartz starting material powder into a rotating mold, forming a crucible-shaped product, arc-fusing the crucible-shaped product to obtain a fused crucible, grinding the entire inner surface of the fused crucible, and heat treating the ground surface with an oxyhydrogen burner has been proposed (Japanese Unexamined Patent Publication (KOKAI) No. 2001-328831 (D 4)).
D 4 states that this method provides a method of manufacturing a silica glass crucible for pulling silicon single crystals that yields a high single crystallization rate without generating dislocations and without entraining bubbles in the single crystals that are pulled even when pulling silicon single crystals.
As set forth above, no silica glass crucible from which all bubbles have been completely removed has yet been achieved. Further, even when the bubbles in the transparent layer are markedly reduced compared to a conventional silica glass crucible, the single crystallization yield of silicon single crystals of which high quality is demanded has not been enhanced to a fully satisfactory degree.
In the methods of manufacturing silica glass crucibles for pulling silicon single crystals described in above-cited D 2 and D4, the entire inner surface is ground to remove bubbles in the transparent layer of the silica glass crucible. However, grinding of the entire inner surface is an operation that requires a considerable amount of time and labor, creating a problem by requiring post-processing to smooth out the surface following grinding.
Accordingly, the present invention has for its object to provide a new method for manufacturing a silica glass crucible for pulling silicon single crystals that reduces the quantity of bubbles in the transparent layer without grinding the entire inner surface.